The present invention relates to electrochromic elements, and more particularly to laminated electrochromic glass devices and processes for making such devices.
Electrochromic materials generally are materials which change color upon application of electrical current to induce an electrochemical reaction in the material.
Electrochromic devices are known which comprise a laminated structure including an electrolyte ion conducting layer sandwiched between an electrochromic layer and a counter electrode layer, all sandwiched between a pair of conductive electrodes composed of, for example, indium-tin oxide.
Many of the prior art electrochromic elements utilize WO.sub.3 as the electrochromic color-forming layer. It is known that WO.sub.3 changes from a clear, transparent state to a colored state upon undergoing the reaction: ##STR1## wherein Z is selected from H or alkali metals such as Li or Na.
It is also known from the nonaqueous secondary battery art that various other metals will display electrochromic properties when changing from one valence state to another. Specifically, it is known that some transition metals exhibit electrochromic properties when moving between the +2 and +3 valence states and other transition metals exhibit such properties when changing between the +3 and +4 valence states.
Heretofore, the art has had difficulty utilizing the electrochromic properties of WO.sub.3 in combination with the electrochromic properties of these other known transition metal oxides. For example, it is disclosed by U.S. Pat. No. 4,750,816 that "oxidatively color-forming materials suitable as opposing electrodes of reductive color-forming layers comprising WO.sub.3, etc. are not found in inorganic materials." Column 1, lines 42-45). This is due to several factors, such as the difficulty in discovering oxidative color-forming materials which (1) have a high enough ion exchange capacity; (2) exhibit an adequate transparency or, even better, exhibit color changes complementary to those of WO.sub.3 ; and (3) have a range of working potential compatible with that of the other materials in the electrochromic element.
The term "complementary" color changes, as used herein, means exhibiting opposite color-forming tendencies upon the insertion of an ion. WO.sub.3 colors when an ion is inserted therein and thus materials "complementary" to WO.sub.3 would bleach upon ion insertion. Thus, an electrochromic element utilizing a layer of WO.sub.3 along with a layer of a material having a complementary color change to WO.sub.3 would have two bleached electrochromic layers when an ion was inserted into the complementary layer and two colored layers when an ion was inserted into the WO.sub.3 layer. This would enable an additive color effect to be attained.
Because of the aforementioned difficulties, prior art electrochromic devices tended to utilize either a single electrochromic layer of WO.sub.3 or other electrochromic material to produce the desired electrochromic color change effect, or utilized either an inorganic compound which undergoes little or no color change upon ion insertion and removal or an organic compound as the opposing or counter electrode to the WO.sub.3 layer. The use of a single electrochromic layer of WO.sub.3 or a layer of WO.sub.3 in conjunction with a counter electrode which remains transparent upon ion insertion and removal, suffers from the disadvantage that the difference in the amount of light that is transmitted through the layer in the clear and colored states is limited by the extent of color change of the WO.sub.3 material. In addition, electrochromic devices utilizing an organic electrochromic layer suffer from the disadvantage that these layers are unstable over long periods of time and thus their long term color-forming durability is questionable.
It is an object of the present invention to provide a novel electrochromic element.
It is another object of the present invention to provide a novel electrochromic glass device.
It is another object of the present invention to provide an electrochromic element which is characterized by having a large difference between the percentage of visible light transmitted by the element in the colored state and the percentage of visible light transmitted by the element in the bleached state.
It is another object of the present invention to provide an electrochromic element which is characterized by having a large difference between the percentage of radiant heat transmitted by the element in the colored state and the percentage of radiant heat transmitted by the element in the bleached state.
It is another object of the present invention to provide an electrochromic element whose color-forming properties exhibit excellent long term durability.
It is another object of the present invention to provide an electrochromic element characterized by having an excellent response time, i.e., the period of time to change between the bleached state and the colored state is low.
It is another object of the invention to provide an electrochromic element that can operate effectively over a wide range of temperatures.
It is a further object of the invention to provide an electrochromic element that does not utilize toxic or corrosive materials.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention.